In some cases, it is necessary to apply at least two superposed layers of different powders, e.g. for vitrified enamel the layers may be as follows:
an undercoat of enamel which contains ingredients for adherence to the support (which is an ordinary metal sheet) and a layer herein called a covering layer of enamel which contains the decorative colouring. A solution known per se consists in placing two powder depositing booths one after the other to carry out this operation.
The difficulties or the complications of this solution are numerous and impede the development of the method:
the undercoat must be very thin for an enamel (about 20 microns), very regular and not very electrostatically charged. This usually requires mechanical equipment moving spray guns vertically in an up and down movement; and
the covering layer must not be polluted by the undercoat which is of a very different chemical nature. Now, in the covering layer depositing booth, the act of depositing said covering layer tends to remove particles of undercoat. Since the non-deposited powder is entirely recycled, the covering layer is polluted thereby.
A known solution consists in depositing the covering layer in two steps, also with up and down movements of spray guns:
step one=deposit recycled powder (only);
step two=deposit new powder (only).
This solution has several disadvantages:
considering the high percentage of powder that is recycled, particularly enamel powder, the quantity of new powder needed to make good powder consumption may be too small to cover properly the layer of polluted recycled powder deposited in the first step; and
the new enamel powder and the recycled enamel powder have different electric characteristics which can cause drawbacks when depositing them separately and, in particular, can lead to a defect called herein"counter-emission".
Preferred implementations of the present invention overcome these disadvantages and complications by providing a method and apparatus which are highly reliable and are inexpensive to provide, in particular by avoiding mechanical equipment of the kind which increases maintenance costs.